3rd Generation (3G) Steels with strengths well above 1GPa has opened up new opportunities for vehicle lightweighting. For many applications high strength must be coupled with sufficient ductility to withstand impact during a crash or forming operations. For stretch forming applications ductility can be adequately characterized by the tensile elongation. However, for forming operations involving bending or out of plane deformation it is the true ductility, i.e. the true strain at fracture, that represents the critical parameter. For some 3G steels true ductility can be remarkable, with fracture strains up to 0.8. This appears to be due to a combination of factors that provide damage tolerant microstructures in these materials. Two primary mechanisms involve grain refinement and TRIP effects, while the mechanical homogeneity of the phases also plays a significant role. With regard to the latter, Figure 1 illustrates the effect in a DP1300 steel to which V has been added. In the V-modified steel the strength of the martensite has been lowered while the ferrite is stronger. This reduction in micromechanical heterogeneity reduces the strain gradient across M-F interfaces making damage nucleation more difficult. The result is a factor of two increase in true ductility.
EXTENDED ABSTRACT